This invention relates to a glass substrate for use in an information recording medium, such as a magnetic disk (namely, a hard disk), an optical disk, and an optical-magnetic disk.
Recently, both higher data transfer rate and higher capacity have been required for a hard disk drive having high performance, such as a server.
Under the circumstance, disk rotation speed has reached 10000 rpm, and further, track density has become 10000 TPI or 2.5 .mu.m between tracks.
In addition, it is also necessary to thin the thickness of the disk in order to stack as much number of disks as possible inside the disk drive having a limited size.
In consequence, high stiffness and/or large damping for reducing flutter has been needed, and further, smoothness for small flying height and surface hardness for higher shock resistance have been also required.
In this condition, an aluminum substrate has been widely used for 3.5-inch disk drive in recent years. However, it is difficult to use the aluminum substrate for the disk drive having high performance because the aluminum substrate has relatively low Young's modulus of about 72 GPa. Further, the surface smoothness and the shock resistance should be significantly improved.
On the other hand, a glass substrates have been mainly used for a 2.5-inch disk drive which are equipped with mobile personal computer (thereinafter, abbreviated to PC). In this event, the disk drive must have much better shock resistance capability as compared with that for a desktop-type PC. Further, higher surface hardness is also required for the glass substrates.
Disclosure is made about a N5 glass substrate, which has been widely used, in Japanese Unexamined Patent Publication (JP-A) Hei 5-32431.
More specifically, the glass substrate is a chemical reinforcement glass which contains 62-57% of SiO.sub.2, 5-15% of Al.sub.2 O.sub.3, 4-10% of Li.sub.2 O, 4-12% of Na.sub.2 O, and 5.5-15% of ZrO.sub.2 by weight %. In this event, the weight ratio of Na.sub.2 O/ZrO.sub.2 falls within the range between 0.5 and 2.0 while the weight ratio of Al.sub.2 O.sub.3 /ZrO.sub.2 falls within the range between 0.4 and 2.5.
The glass substrate has excellent shock resistance more than 600 G. However, the Young's modulus is 83 GPa that is slightly larger than the aluminum substrate.
In the meanwhile, glass-ceramics substrates have been also introduced into the disk drive market. For example, this glass-ceramics substrate has been disclosed in U.S. Pat. No. 5,391,522. Some of the glass ceramics substrates often have higher Young's modulus.
More specifically, the glass substrate contains 65-83% of SiO.sub.2, 8-32% of Li.sub.2 O, 0-7% of K.sub.2 O, 0.5-5.5% of MgO, 0-5% of ZnO, 0-5% of PbO, (herein, 0.5-5% of MgO+ZnO+PbO), 1-4% of P.sub.2 O.sub.5, 0-7% of Al.sub.2 O.sub.3, 0-2% of As.sub.2 O.sub.3 +Sb.sub.2 O.sub.3 by weight %, and contains fine Li.sub.2 O.2SiO.sub.2 crystal grains as a main crystal.
However, the above-mentioned grass-ceramics substrate has a large crystal grain. As a result, it is difficult to smooth the surface of the substrate. Therefore, large cost is needed so as to smooth the surface.
As mentioned before, the conventional disk substrate does not sufficiently satisfy all of the small surface smoothness, the shock resistance, the high stiffness and/or the large damping for the hard disk having high performance, such as the server.